(tanα-tanβ)/(tanα+tanβ)
=(sinα/cosα-sinβ/cosβ)/(sinα/cosα+sinβ/cosβ)（切化弦）
=(sinacosβ-cosαsinβ)/(sinacosβ+cosαsinβ)
=sin(α-β)/sin(α+β)
=(2/5)/(2/3)
=3/5